MECHANICAL VENTILATION.
By mechanical ventilation we mean the use of machinery to compel the uniform movement of a column of air through the avenues and workings of the mine. This is done by various means. That in use was the ordinary fan driven by hand or ma chinery to propel or blow a current of air through certain portions of the mine, or where natural currents could not be maintained. This mode has also been tried for the pur pose of extensive ventilation, but did not answer. The difficulty of pushing or forcing a current of air through long, intricate, and rough passages is manifest, and both theory and practice are against it.
Air is an elastic fluid, that increases in tension by the pressure exerted. Air occu pying a certain space under the ordinary atmospheric pressure can be reduced to half the space or bulk by double the pressure; and to one-third the space under a treble pressure, and so on: consequently, the density or weight of a given column of air varies directly as the pressure on each unit of surface under which it exists, the tem perature remaining unchanged.
The effect, therefore, of forcing air through a long series of intricate passages is to increase its density and friction in proportion to the pressure applied and the length of the column. To a limited extent this may be done by the expenditure of suffi cient power, but this may be compared to the attempt to push a rope instead of pulling it. Whether the ordinary blowing-fan or blowing-cylinder be used, the difficulties are the same: therefore this mode must be condemned. But when the same power is reversed, and the fan or cylinders are made to draw or suck the air instead of pushing it, the effect is reversed, and the natural or atmospheric pressure becomes an active agent instead of a repellant force; instead of increasing the density and consequent friction of the air, it decreases them, and proportionately decreases the amount of power required to produce a given volume or current of air.
We have seen that 109 horse-power produces 175,000 cubic feet of air per minute by the furnace and steam jets in the Hetton colliery; and this is one mode of drawing a column of air. To propel the same column through the same avenues by any blowing
process would require an expenditure of ten times the power, or 1090 horse-power, which would also so increase the density of the air that it would be extremely diffi cult to prevent its escape through the doors and divisions of the air-passages to the return air-course before it could arrive at the extremities of the mine.
The expenditure of power, however, is greater by the furnace mode than by the mechanical or suction mode. Lemielle's and Guibal's apparatus for exhausting the impurities of the mine, though clumsy, costly, and ponderous, is more effective than the furnace. These machines produce about 75,000 cubic feet per minute, under an expenditure of 50 horse-power. This, however, is less than the result of the Hetton experiments, which were more favorable than ordinary; and, generally, it is conceded that even Lemielle's ponderous machines are more reliable, safe, and economical than furnaces. There were several of these machines in use in France in 1857-58, and about this time Nasmyth, of England, applied an improved fan as an exhauster in England; but up to 1863-64 they were not in use in the English collieries, with rare exceptions. They had not been introduced by the mining engineers of the North of England at that time, but were under discussion, and generally conceded to be superior to the furnace and more available.
The use of the fan as an exhauster appears to have been first successfully applied in the anthracite region at Locustdale, near Ashland, Pennsylvania, by John Louden Beadle, a mining engineer of much practical experience and ability. It was introduced in 1857-58, after a long series of practical experiments in blowing and exhausting the air, and was adopted as the result of these experiments. This fan, as applied, is the simplest in use and the most effective. With half the expenditure of power and means required in the French suction apparatus, it produces better results.